1. Field of the Invention
The invention relates to an image synthesizing method for forming a composite image from one or more basic images, each of which is acquired by means of a respective sensor having a sensitivity which varies across the area to be imaged, the image values of the composite image being derived from the image values of the basic image, or basic images, weighted in dependence on the sensitivity.
2. Description of the Related Art
An image synthesizing method of this kind can be used notably for MR examinations intended to determine the nuclear magnetization distribution in an examination zone, the MR signals generated in the examination zone then being picked up by a plurality of surface coils which are distributed around the examination zone and whose data is first separately processed and converted into basic images. In comparison with an MR examination utilizing only one receiving antenna having a constant sensitivity across the entire area to be imaged, the advantage of an enhanced signal-to-noise ratio is thus achieved. However, the sensitivity of such surface coils is strongly dependent on the location, so that the basic images determined for the individual coils cannot be simply added so as to obtain a composite image which optimally reproduces the entire examination zone.
An image synthesizing method of the kind set forth is already known from an article by Roemer et al. in Magn. Reson. Med. 16, pp. 192-225 (1990); therein, a composite image is formed from a plurality of basic images in conformity with the relation EQU P.sub.jk =p.sub.j.sup.t.sub.k R.sup.-1 b*.sub.jk ( 1)
Therein:
P.sub.jk is a complex image value of the composite image for a pixel situated in row j and column k of the composite image, PA1 p.sup.t.sub.jk is the transposed form of a column vector p.sub.jk which is composed of the complex image values of the individual basic images for the pixel jk PA1 R.sup.-1 is the inverse of the so-called noise correlation matrix (called a "noise resistance matrix" in the article by Roemer), and PA1 b*.sub.jk is the conjugate complex form of a column vector b.sub.jk which is composed of the values of the sensitivities of the individual sensors for the pixel jk.
However, the effect of the location-dependent sensitivity is still noticeable, in the image values P.sub.jk of the composite image thus calculated, so that the points jk in the composite image which are situated furthest from the individual sensors or coils have the lowest brightness. The effect of the location-dependency of the sensor sensitivities can be eliminated according to Roemer as follows: ##EQU1## Therein, b.sup.t.sub.jk is the transposed form of the sensitivity vector b.sub.jk.
If this calculation rule is used in practice, a problem is encountered in that b.sub.jk, or the sensitivity of the individual sensors, at the various pixels of the examination zone is not known.
EP-OS 695 947 describes an MR method in which the sensitivity of the individual surface coils is determined by means of an additional coil which has an approximately constant sensitivity for the entire examination zone. During temporally separated steps MR images are then formed for this coil on the one hand and for the surface coils on the other hand, a measure of the sensitivity of the surface coil in the relevant pixel being derived from the ratio of an image value of the additional MR coil to an image value of a surface coil for the same pixel. Using the complex values of the sensitivity and the image values of the basic images of the individual coils thus obtained, a synthesizing operation is performed in conformity with the equations given in the cited publication, thus forming a composite image from the basic images. It is a drawback, however, that an additional MR coil and additional measurements are required.
Knowledge of the spatial variation of the sensitivity is important also if a basic image picked up by a single sensor is to be reproduced in such a manner that the location-dependent sensitivity of this sensor does not become manifest as a location-dependent brightness difference.